Furnace for reheating slabs or billets

ABSTRACT

A reheating furnace for steel slabs or billets includes a plurality of generally parallel skids extending longitudinally of the furnace, a plurality of heat sources within the furnace and electric induction heaters associated with the skids for applying additional heat to the under surfaces of the workpieces. The induction heaters comprise a plurality of cores having electrically inductive windings thereon, each core being recessed in the soaking hearth of the furnace below a layer of refractory material and extending longitudinally of the hearth substantially in alignment with a respective skid.

United States Patent [191 Laws Nov. 26, 1974 [54] FURNACE FOR REHEATING SLABS OR 3,596,036 7/1971 Ross 2l9/l0.69 BILLETS [75] Inventor: William Robert Laws, Worcester Primary Examiner-R. N. Envall, Jr.

Park, England Attorney, Agent, or FirmBac0n & Thomas [73] Assignee: British Steel Corporation, London,

England [57] ABSTRACT [22] Filed: Oct. 26, 1973 A reheating furnace for steel slabs or billets includes a l [2 1 App! NO 410 273 plurality of generally parallel skids extending longitudinally of the furnace, a plurality of heat sources [30] Foreign Application Priority Data within the furnace and electric induction heaters asso- Nov. 10. 1972 Great Britain 52047 72 Ciated with the s s r pplying dditiona h at to the under surfaces of the workpieces. Cl l9/10.69, 4 The induction heaters comprise a plurality of cores having electrically inductive windings thereon, each lilt- Cl. Core being recessed in the oaking hearth of the Fleld of Search furnace below a layer of refractory material and 432/127 extending longitudinally of the hearth substantially in alignment with a respective skid. [56] References Cited UNITED STATES PATENTS 13 Claims, 3 Drawing Figures 3,342,468 9/1967 Sidwell 432/127 PATENTL ZQUVZB 1974 3,851,091 SHEET 2 OF 2 FURNACE FOR HEATING SLABS OR BILLETS This invention relates to a furnace for reheating metal workpieces, particularly steel slabs or billets.

In particular the invention relates to a top and bottom fired pusher reheating furnace wherein the length of the soaking hearth is very short in relation to the overall length of the furnace. A reheating furnace having such a short soaking hearth is described in US. Pat. No. 3,623,715, Although said patent relates primarily to an opposed zone furnace, the present invention is not thus limited in its application, but is of interest in any reheating furnace having a relatively short soaking hearth as above mentioned.

It is possible to utilise a short soaking hearth in a metal slab reheating furnace if the furnace is sufficiently efficient to have provided substantially homogeneous heat distribution throughout the metal slab or billet at the time that the slab or billet leaves the top and bottom heating zones of the furnace. Indeed under ideal furnace design conditions it would be possible to dispense absolutely with a soaking hearth since, in an ideal furnace, the slabs or billets would always have achieved homogeneous heat distribution throughout on their emergence from the tonnage zone.

One of the means of achieving substantially homogenous heat distribution throughout the slab or billet is to employ a skid system so designed as to minimise the likelihood of occurrence of skid marks on the under surface of the slab or billet. Part of the skid system described and illustrated in the aforesaid British patent employs substantial insulation around the skid pipe, and if such insulation degrades or becomes detached from the skid system during use of the furnace, then there will be a tendency towards the development of skid marks on the under surface of the slab or billet supported on the skid rails. However, in a furnace having no soaking hearth, or a relatively short soaking hearth, it will not be possible for the slabs or billets to achieve homogeneity of heat distribution following the occurrence of skid marks on their under surfaces.

It is an object of the present invention to provide a furnace for heating metal workpieces wherein the formation of skid marks in the workpiece can be alleviated before the workpieces leave the furnace.

In accordance with one aspect of the invention there is provided a heating furnace for metal workpieces comprising a plurality of skids extending longitudinally of the furnace for supporting metal workpieces as they pass through the furnace; a plurality of heat sources located within the furnace and electric induction heating vfor the workpieces as they pass through the hearth from the skids. The induction heating means may comprise a plurality of cores having electrically inductive windings thereon, each core extending longitudinally of the soaking hearth substantially in alignment with a respective skid.

In accordance with a further aspect of the invention there is provided a method of heating metal workpieces during their passage through a heating furnace on a plurality of skids extending longitudinally of the furnace comprising applying heat to the workpieces from a plurality of heat sources within the furnace and applying heat to the under surfaces of the workpieces, additional to the heat supplied by said heat sources, by electric induction heaters associated with the skids.

The additional heat may be applied to the under surfaces of the workpieces downstream of the skids in a soaking hearth in zones extending longitudinally of the hearth substantially in alignment with respective skids.

A furnace in accordance with the invention conveniently comprises a reheating furnace for steel slabs or billets.

One example of a heating furnace constructed in accordance with the invention will now be described solely by way of example and with reference to the accompanying drawings wherein:

FIG. l is a longitudinal cross sectional view through a reheating furnace in accordance with the invention,

FIG. 2 is an enlarged perspective detail, partly in section, of part of the soaking hearth and skid rails, and

FIG. 3 is a detail transverse cross sectional view through part of the soaking hearth showing one of the induction heaters in position.

The furnace illustrated in the drawings is designed for reheating steel slabs or billets S and comprisesa top and bottom fired five zone pusher furnace of opposed zone design. The furnace has an entry 10 to the preheating zones L2 and an exit 14 downstream of the soaking hearth 16 leading to a discharge ramp 18 and door 20. The preheating zones 12 are provided with burners 12a arranged so that the flames travel in the same direction as that of the slabs or billets, whilst the tonnage zones 22 and soaking hearth B6 are provided with burners 22a and 16a respectively arranged so that their respective flames travel in the opposite direction to that of the slab or billet. A waste gas off-take 24 is disposed between the preheating zones 12 and the tonnage zones 22.

The furnace is of the type known as a pusher furnace having a skid system 26 for supporting the slabs or billets S as they are pushed in succession through the furnace in a broadside attitude from the entry 10 to the discharge door 20 thereof. Although illustrated diagrammatically herein as skids 26 it will be appreciated that the skid system may be of any suitable form. It will also be appreciated that the furnace itself, although described herein as a five zone opposed system, may not be thus limited in the application of the present invention.

As will be clearly seen from FIG. 1 of the drawings the preheating zones 12 are considerably longer than the tonnage zones 22, whilst the soaking hearth I6 is itself only approximately half the length of the tonnage zones 22. The length of the soaking hearth 16 is thus very short in relation to the overall length of the furnace.

It will be seen from FIGS. 1 and 2 that the skid system is illustrated as comprising a plurality of skid rails 26 extending longitudinally of the furnace in spaced paralle] relationship with one another through the preheating and tonnage zones 12 and 22; said skid rails 26 terminating at the soaking hearth 16 so that the slabs or billets may be pushed in a broadside attitude on to the soaking hearth support surface and subsequently through the hearth down the'discharge ramp 18 and out of the discharge door 20. In accordance with one aspect of the invention a plurality of electric induction heaters 28 are provided in the soaking hearth 16 in alignment with the skid rails 26. The induction heaters 28 are of elongated configuration extending preferably for the full length of the soaking hearth 16 in spaced parallel relationship with one another in alignment with the spaced parallel skids 26. Each induction heater 28 comprises a multi-layer winding 30 around a plurality of E-shaped iron laminations constituting an E-shaped core 32 to each heater. As may be clearly seen from FIGS. 2 and 3 the aforesaid core 32 is recessed and orientated with the refractory material 17 of the soaking hearth 16 in such manner that the ends of the limbs 32a of the core 30 are all co-planar and flush with the upper surface of the refractory hearth material 17. In order to avoid damage to the hearth a plane surface of refractories 34 is laid over the hearth material 17 covering the heaters 28.

The induction heaters would normally be water cooled but details of the cooling system have been omitted herein and from the drawings for the sake of clarity.

The electric induction heaters 28 are of the type known as transverse flux inductors which produce calised bands of heat in a ferro-magnetic workpiece. Such localised bands of heat can thus be applied to the under surface of a steel slab or billet S whilst it is in the soaking hearth 16 of the furnace.

It will be appreciated therefore that if for some reason skid marks are formed in the under surface of the slabs or billets S during their passage through the preheating and tonnage zones 12 and 22, then such skid marks, which are in fact cold bands, can be erased by applying localised band heating from the electric inductors 28. Since it is a known phenomenon that electrical conductivity decreased with temperature rise, it will be apparent that the initial heating of the cold band skid mark will be concentrated but that the heating effect will spread around the skid marked areas through the slabs or billets at higher temperatures. It is envisaged therefore that the induction heaters could be utilised to heat the whole of the slab or billet during its time in the soaking hearth in the event of a complete stoppage of throughput in the furnace.

The foregoing description is related to the association of the induction heaters with the skids by aligning the heaters with respective skids in the soaking hearth. However, it is contemplated, although not illustrated herein, that the electric induction heaters may be associated with the skids for applying localised heating to the under surfaces of the slabs or billets as hereinbefore described by positioning the heaters directly adjacent or within, or forming part of, the skid system itself.

I claim:

1. A heating furnace for metal workpieces comprising a plurality of skids extending longitudinally of the furnace for supporting metal workpieces as they pass through the furnace; a plurality of heat sources located within the furnace and electric induction heating means located below the level of the tops of the skids for applying heat in discrete zones extending longitudinally of the furnace solely to the undersurfaces of the workpieces additional to the heat provided by said heat sources.

2. A heating furnace as claimed in claim 1 wherein said induction heating means are located within the furnacle downstream of said skids and in alignment therewit 3. A heating furnace as claimed in claim 1 wherein a soaking hearth is provided downstream of said skids and said induction heating means are recessed within said hearth extending longitudinally thereof substantially in alignment with said skids.

4. A heating furnace as claimed in claim 3 wherein said induction heating means are covered with refractory material providing a continuous bearing surface in the hearth for the workpieces as they pass through the hearth from said skids.

5. A heating furnace as claimed in claim 3 wherein said induction heating means extend longitudinally from end to end of the soaking hearth.

6. A heating furnace as claimed in claim 3 wherein said induction heating means comprise a plurality of cores having electrically inductive windings thereon, each core extending longitudinally of the soaking hearth substantially in alignment with a respective skid.

7. A heating furnace as claimed in claim 1 comprising a reheating furnace for steel slabs or billets.

8. A method of heating metal workpieces during their passage through a heating furnace on a plurality of skids extending longitudinally of the furnace comprising applying heat to the workpieces from a plurality of heat sources within the furnace and applying heat, additional to the heat supplied by said heat sources, in discrete zones extending longitudinally of the furnace solely to the undersurfaces of the workpieces by electric induction heaters located below the level of the tops of the skids.

9. A method according to claim 8 wherein the additional heat is applied to the under surfaces of the workpieces downstream of the skids.

10. A method according to claim 9 wherein the additional heat is applied to the under surfaces of the workpieces in a soaking hearth, downstream of the skids, in zones extending longitudinally of the hearth substantially in alignment with respective skids.

11. In a heating furnace having a heating zone and a soaking zone for reheating metal workpieces, a plurality of skids in the heating zone extending longitudinally of the furnace for supporting metal workpieces as they pass through the furnace heating zone, and a plurality of heat sources located within the furnace for heating metal workpieces passing therethrough, the improvement comprising: a plurality of elongated, transverse flux-type, electric induction heaters located in the soaking zone below the level of the tops of the skids and disposed in space, parallel relationship in alignment with the skids for applying heat in localized bands to the portion of the undersurface of the workpieces which supported them on the skids during their passage through the heating zone.

12. A heating furnace according to claim 11 wherein each of the transverse flux-type electric induction heaters comprises an elongated, generally E-shaped core with the limbs of the core extending vertically upwardly from the body portion of the core, and with multi-layer windings disposed in the space between said limbs.

13. A heating furnace according to claim 12 wherein said cores are covered with refractory material providing a continuous load bearing surface in the soaking zone for said workpieces. 

1. A heating furnace for metal workpieces comprising a plurality of skids extending longitudinally of the furnace for supporting metal workpieces as they pass through the furnace; a plurality of heat sources located within the furnace and electric induction heating means located below the level of the tops of the skids for applying heat in discrete zones extending longitudinally of the furnace solely to the undersurfaces of the workpieces additional to the heat provided by said heat sources.
 1. A heating furnace for metal workpieces comprising a plurality of skids extending longitudinally of the furnace for supporting metal workpieces as they pass through the furnace; a plurality of heat sources located within the furnace and electric induction heating means located below the level of the tops of the skids for applying heat in discrete zones extending longitudinally of the furnace solely to the undersurfaces of the workpieces additional to the heat provided by said heat sources.
 2. A heating furnace as claimed in claim 1 wherein said induction heating means are located within the furnace downstream of said skids and in alignment therewith.
 3. A heating furnace as claimed in claim 1 wherein a soaking hearth is provided downstream of said skids and said induction heating means are recessed within said hearth extending longitudinally thereof substantially in alignment with said skids.
 4. A heating furnace as claiMed in claim 3 wherein said induction heating means are covered with refractory material providing a continuous bearing surface in the hearth for the workpieces as they pass through the hearth from said skids.
 5. A heating furnace as claimed in claim 3 wherein said induction heating means extend longitudinally from end to end of the soaking hearth.
 6. A heating furnace as claimed in claim 3 wherein said induction heating means comprise a plurality of cores having electrically inductive windings thereon, each core extending longitudinally of the soaking hearth substantially in alignment with a respective skid.
 7. A heating furnace as claimed in claim 1 comprising a reheating furnace for steel slabs or billets.
 8. A method of heating metal workpieces during their passage through a heating furnace on a plurality of skids extending longitudinally of the furnace comprising applying heat to the workpieces from a plurality of heat sources within the furnace and applying heat, additional to the heat supplied by said heat sources, in discrete zones extending longitudinally of the furnace solely to the undersurfaces of the workpieces by electric induction heaters located below the level of the tops of the skids.
 9. A method according to claim 8 wherein the additional heat is applied to the under surfaces of the workpieces downstream of the skids.
 10. A method according to claim 9 wherein the additional heat is applied to the under surfaces of the workpieces in a soaking hearth, downstream of the skids, in zones extending longitudinally of the hearth substantially in alignment with respective skids.
 12. A heating furnace according to claim 11 wherein each of the transverse flux-type electric induction heaters comprises an elongated, generally E-shaped core with the limbs of the core extending vertically upwardly from the body portion of the core, and with multi-layer windings disposed in the space between said limbs.
 13. A heating furnace according to claim 12 wherein said cores are covered with refractory material providing a continuous load bearing surface in the soaking zone for said workpieces. 